package org.blankhole.filter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

/**
 * TemplateFilter: 模板过滤器，提供公用方法的默认实现，并定义预处理(
 * {@link #before(ServletRequest, ServletResponse)})、后处理(
 * {@link #after(ServletRequest, ServletResponse)})接口，子类根据需要实现即可。
 * 
 * @author Wenlong Meng(wenlong.meng@gmail.com)
 * @version 1.0 at 2012/08/14
 * @since 1.0
 */
public abstract class TemplateFilter implements Filter {

	// Local variables
	/**
	 * filter config
	 */
	private FilterConfig filterConfig;

	// Logic
	/**
	 * Called by the web container to indicate to a filter that it is being
	 * taken out of service. This method is only called once all threads within
	 * the filter's doFilter method have exited or after a timeout period has
	 * passed. After the web container calls this method, it will not call the
	 * doFilter method again on this instance of the filter. <br>
	 * <br>
	 * 
	 * This method gives the filter an opportunity to clean up any resources
	 * that are being held (for example, memory, file handles, threads) and make
	 * sure that any persistent state is synchronized with the filter's current
	 * state in memory.
	 * 
	 * @see javax.servlet.Filter#destroy()
	 */
	@Override
	public void destroy() {
		// default: do nothing
	}

	/**
	 * The <code>doFilter</code> method of the Filter is called by the container
	 * each time a request/response pair is passed through the chain due to a
	 * client request for a resource at the end of the chain. The FilterChain
	 * passed in to this method allows the Filter to pass on the request and
	 * response to the next entity in the chain.
	 * <p>
	 * A typical implementation of this method would follow the following
	 * pattern:- <br>
	 * 1. Examine the request<br>
	 * 2. Optionally wrap the request object with a custom implementation to
	 * filter content or headers for input filtering <br>
	 * 3. Optionally wrap the response object with a custom implementation to
	 * filter content or headers for output filtering <br>
	 * 4. a) <strong>Either</strong> invoke the next entity in the chain using
	 * the FilterChain object (<code>chain.doFilter()</code>), <br>
	 * 4. b) <strong>or</strong> not pass on the request/response pair to the
	 * next entity in the filter chain to block the request processing<br>
	 * 5. Directly set headers on the response after invocation of the next
	 * entity in the filter chain.
	 * 
	 * @see javax.servlet.Filter#doFilter(javax.servlet.ServletRequest,
	 *      javax.servlet.ServletResponse, javax.servlet.FilterChain)
	 **/
	@Override
	public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain filterChain) throws IOException, ServletException {
		// Preprocessing for each filter
		before(request, response);

		// Pass control to the next filter in the chain or
		// to the target resource. This method invocation is what logically
		// demarcates preprocessing from postprocessing.
		filterChain.doFilter(request, response);

		// Post-processing for each filter
		after(request, response);
	}

	/**
	 * pre processing before execute
	 * 
	 * @param request
	 * @param response
	 */
	public abstract void before(ServletRequest request, ServletResponse response);

	/**
	 * post processing after execute
	 * 
	 * @param request
	 * @param response
	 */
	public abstract void after(ServletRequest request, ServletResponse response);

	/**
	 * Called by the web container to indicate to a filter that it is being
	 * placed into service. The servlet container calls the init method exactly
	 * once after instantiating the filter. The init method must complete
	 * successfully before the filter is asked to do any filtering work. <br>
	 * <br>
	 * 
	 * The web container cannot place the filter into service if the init method
	 * either<br>
	 * 1.Throws a ServletException <br>
	 * 2.Does not return within a time period defined by the web container
	 * 
	 * @see javax.servlet.Filter#init(javax.servlet.FilterConfig)
	 */
	@Override
	public void init(FilterConfig filterConfig) throws ServletException {
		this.filterConfig = filterConfig;
	}

	/**
	 * get filter config for subclass
	 * 
	 * @return filterConfig
	 */
	protected FilterConfig getFilterConfig() {
		return filterConfig;
	}

}
